Prep your pans and ingredients: Line two baking sheets with parchment paper. Measure all ingredients. If using chocolate chunks, chop them now.

Mix dry ingredients: In a medium bowl, whisk 2 cups all-purpose flour, 1/2 cup unsweetened cocoa powder, 1/2 cup hot cocoa mix, 1 teaspoon baking soda, and 1/2 teaspoon fine sea salt. If using, add 1/2 teaspoon espresso powder.

Combine sugars and butter: In a large bowl, whisk 3/4 cup melted unsalted butter (slightly cooled) with 3/4 cup light brown sugar and 1/2 cup granulated sugar until glossy and well blended, about 1 minute.

Add eggs and vanilla: Whisk in 2 large eggs, one at a time, followed by 2 teaspoons vanilla extract. Mix until the batter looks smooth and slightly thick.

Bring wet and dry together: Add the dry mixture to the wet mixture. Stir with a spatula until just combined. Do not overmix—stop when you no longer see streaks of flour.

Fold in chocolate: Gently fold in 1 1/2 cups semi-sweet or dark chocolate chips/chunks. The dough will be thick and glossy.

Chill: Cover and refrigerate for 30–45 minutes. This step helps prevent spreading and keeps the centers soft.

Preheat and scoop: Preheat the oven to 350°F (175°C). Scoop 2-tablespoon mounds of dough (a medium cookie scoop works well) and space them 2 inches apart on the prepared sheets. Roll into balls for a tidy shape if you like.

Bake, then add marshmallows: Bake for 7–8 minutes. Quickly pull out the tray and press 3–5 mini marshmallows on top of each cookie. Return to the oven and bake 2–3 more minutes, until the edges are set and the centers are puffy but soft.

Optional finish: While warm, sprinkle a pinch of flaky sea salt over each cookie to balance the sweetness.

Cool: Let the cookies rest on the baking sheet for 5 minutes, then transfer to a wire rack to cool. They will deflate slightly as they set.